The morphology and postnatal development of axonal terminal arbors will be studied. Once axons have reached their target, the sequence of structural events that occur to produce adult forms will be documented. Axons will be injected close to their terminations with horseradish peroxidase (HRP). The three-dimensional forms of terminal arbors and details of their appendages will be characterized in the light microscope and synaptic patterns of terminals will be identified in the electron microscope. Retino-geniculate axons in the kitten and cerebellar mossy and climbing fibers in postnatal mice will be studied with respect to their branching pattern as they expand and mature. The types of growing tips will be identified and their placement on the arbor, cytological features, and presence of specialized contacts will be described. Particular attention will be given to the refinement of mature shapes of terminal swellings and the corresponding development of synaptic relations. The structural changes accompanying potential withdrawal of supernumerary branches, as well as reduction of terminals will be documented. The development of shape of dendritic appendages of postsynaptic cells will also be followed in Golgi preparations to elucidate the relationship of axonal development to dendritic form. These studies should provide a morphological basis for development of axons in general and for future experiments utilizing surgical intervention, genetic mutants, or environmental manipulation.